I find that some Duplicate meta descriptions/Duplicate title tags in my google webmaster tools. Does it affects my position?
Thank you.

Yes , Duplicate Title Tags May effect your rankings , as google wants every page to has its own genuine and relevant Title's
or in simple words you can understand as every page has its own importance Like services.html will describe about services being provided by that company , in aboutus.html it will be about the company itself etc , As every page has its own importance it is understood that each page has its own title's etc.

Yes absolutely it matters. About 4 months ago my site faced this small issue but as since been fixed. If you have dup title and desc even though the url is different google will automatically choose which to include in SERPs.

My site had the article page and any comments for that review. meta tags were the same so one day this url would show up in the results, the next a different one. Rule of thumb.. uniqueness is the key.

you should Create unique title tags for each page.Each of your pages should ideally have a unique title , which helps Google to know how the page is distinct from the others on your site. Titles can be both short and informative but using extremely lengthy titles that are unhelpful to users stuffing unneeded keywords in your title tags.It also stop our site to attain higher page rank.

Duplicate title tags and meta descriptions do effect the position of a website. As Google wants unique content on every website so also it wants the title and meta descriptions tags to be unique so that it can easily recognize the pages and will provide better SERPs for the relevant search queries.